7292 Macromolecules 2009, 42, 7292–7298 DOI: 10.1021/ma901323f

Fast Access to Dendrimer-like Poly()s through Anionic Ring-Opening of Ethylene Oxide and Use of Nonprotected as Branching Agent

Xiaoshuang Feng,† Daniel Taton,*,† Elliot L. Chaikof,‡ and Yves Gnanou*,† †Universite Bordeaux, Laboratoire de Chimie des Polymeres Organiques, ENSCPB-CNRS 16, Avenue Pey Berland, 33607 Pessac cedex, France, and ‡Laboratory for Biomolecular Materials Research, Department of Surgery, Emory University School of Medicine, Atlanta, Georgia 30322 Received June 19, 2009; Revised Manuscript Received August 28, 2009

ABSTRACT: Dendrimer-like poly(ethylene oxide)s (PEOs) were synthesized through a semicontinuous process based on the anionic ring-opening polymerization (AROP) of ethylene oxide (EO), followed by AROP of a mixture of glycidol (G) and propylene oxide (PO). Glycidol was used as branching agent generating two hydroxyl groups after ring-opening, whereas propylene oxide served to prevent the aggregation of the generated terminal alkoxides. A three-armed PEO star was first prepared through AROP of EO from 1,1,1-tris(hydroxymethyl)ethane as trifunctional precursor using dimethyl sulfoxide (DMSO) as . After completion of EO polymerization and without isolating the PEO star precursor, G and PO (molar ratio 1:3) were added in the same batch to be polymerized either sequentially or randomly. This led to a three-armed PEO star with an average number of terminal hydroxyls per arm which depended on the number of G units inserted at PEO chain ends, as determined by 1H NMR spectroscopy. Growth of the second and the third generation of PEO could be achieved upon reiterating the same steps of AROP of EO and subsequent AROP of G and PO (arborization step) in one pot, affording dendrimer-like PEOs of generation 3 with moderately distributed but expected molar masses. In a variant of this strategy, G was copolymerized in the presence of allyl glycidyl during the arborization step in order to introduce allylic double bonds at the branching points of the dendrimer-like PEOs.

Introduction functions, but possess true oligomeric/polymeric segments linking Widely acknowledged as the reference biocompatible polymer their branching points. The advantages provided by the dendritic used in the biomedical field, poly(ethylene oxide) (PEO), also structure, in particular its multivalency, can be combined with the referred to as poly(ethylene glycol) (PEG), possesses unique unique features of polymers, in the case of PEO its stealth effect. properties such as chemical stability, in both organic From a synthetic point of view, dendrimer-like PEOs are and aqueous media, nontoxicity, low immunogenicity, and anti- synthesized by repeating two elementary steps that are the anionic genicity.1 PEG has found numerous applications as conjugates to ring-opening polymerization (AROP) of ethylene oxide (EO) from multifunctional precursors, followed by the arborization biologically active molecules/substrates (PEGylation), providing 30,31 the latter with a protection from the immune system and with an (branching reaction) of PEO chain ends. However, this enhanced circulation time and efficacy.1-3 Applications of PEG branching reaction always requires the specific design of branch- ing agents, making the synthesis of dendrimer-like PEOs tedious often use R,ω-heterodifunctional oligomers possessing end func- 21-31 tional groups such as amine, carboxylic, thiol, aldehyde, or as that of regular dendrimers that is achieved stepwise. maleimide to react with a variety of ligands.1,4-6 However, low In this contribution, we explore a shorter divergent synthetic linear PEG precursors with a limited attachment pathway that would allow a fast access to dendrimer-like PEOs capacity are generally employed. Arranging PEG chains into a by directly using glycidol (G) as a branching agent. Nonprotected branched architecture carrying many reactive sites might offer G leads to polydisperse hyperbranched polyether polyols, re- better performance in biomedical applications than with linear ferred to as hyperbranched polyglycerols (HPG), by ring-opening 2,3,7,8 multibranching polymerization (ROMBP). The seminal work by homologues. Branched PEOs possessing a high number of 32 peripheral reacting sites include star-like,7-15 hyperbranched,16,17 Vandenberg et al. has been further revisited by Mulhaupt, Frey, arborescent,18-20 or dendrimer-like PEOs.21-29 Haag, and others who developed a slow monomer addition In recent years, we have developed synthetic strategies leading to process and the use of partially deprotonated multihydroxy- multihydroxy-ended,22,23 pH-sensitive,24 heterodifunctional bou- containing core precursor, in order to achieve better-defined 25 26 materials in terms of molar masses, degree of branching, and quet-type, and asymmetrical Janus-type dendrimer-like PEOs 33-40 and to their applications in the biomedical field. Generally speak- molar masses distribution. Here we make use of G for the ing, dendrimer-like polymers exhibit structural features similar to arborization of PEO chains without resorting to any protection/ those of regular dendrimers, including the presence of a central deprotection steps. A mixture of propylene oxide (PO) and G is core, a precise number of branching points, and outer terminal indeed directly added at the completion of AROP of EO. The role of PO is to minimize the aggregation of the alkoxides, whereas G generates two hydroxyls through its ring-opening. The number of *Corresponding author. E-mail: [email protected] (D.T.), gnanou@ peripheral hydroxyls and hence the average branching multi- enscpb.fr (Y.G.). plicity are given by the degree of polymerization of the short HPG pubs.acs.org/Macromolecules Published on Web 09/08/2009 r 2009 American Chemical Society Article Macromolecules, Vol. 42, No. 19, 2009 7293

Table 1. Molecular Characteristics of Dendrimer-like PEOs of Second Generation and of Corresponding Precursors Obtained Using Glycidol as Branching Agent

a c d f 3 g 3 sample conv (%) N(OH) Mn,theo (10 ) Mn,NMR (10 ) PDI

G1a-[HPGNPPO] 60 5.5 5.96 5.20 1.05 G2a 5.5 11.3 11.5 1.06 G1b-[HPGNPPO] 72 12.9 7.83 6.70 1.08 G2b 12.5 13.7 19.9 1.10 G1c-[HPGNPPO] 74 18.2 9.34 7.97 1.10 G2c 18.4 26.2 30.0 1.11 b e G1d-[HPGNPPO] 80 15.3 (2.8) 7.19 5.60 1.12 G2d 14.7 (2.7)e 20.6 17.1 1.12 a In these notations, the number 1 or 2 represents the generation number, and the letters a to d are meant to distinguish PEO samples of a same series. b Glycidol, propylene oxide, and allyl glycidyl ether were introduced as a mixture in the reaction medium; the other samples G1-[HPGNPPO] (a to c) were prepared through sequential addition technique. c The conversion of glycidol calculated based on the amount of glycidol added. d Number of periphery OH groups generated by arborization of PEO chains and calculated using eq 1. e Average number of allyl glycidyl ether units incorporated. f g Mn calculated based on the amount of the added monomers. Determined using eq 2.

Table 2. Molecular Characteristics of Dendrimer-like PEOs of Second and Third Generation Obtained Using Glycidol as Branching Agent

a c d f 3 g 3 sample conv (%) N(OH) Mn,theo (10 ) Mn,NMR (10 ) PDI

G2a-[HPGN0PPO] 65 22.0 17.6 14.3 1.24 G3a 24.2 36.3 35.3 1.36 b G2b-[HPGN0PPO] 74 47.3 31.0 26.1 1.38 G3b[N0] 51.1 73.4 77.4 1.30 G2b-[HPGN00PPO] 64 49.5 33.9 24.8 1.27 G3b[N00] 57.0 74.3 77.4 1.18 b G2c-[HPGN0PPO] 71 74.0 49.1 45.5 1.18 G3c 78.0 120 131 1.34 e G2d-[HPGN0PPO] 73 45.6 (7.6) 32.8 20.1 1.28 G3d 47.5 (7.6)e 65.7 56.6 1.31 a See Table 1 for notation. b A mixture of glycidol, propylene oxide, and allyl glycidyl ether was introduced as a mixture in the reaction medium; c samples G2-[HPGN0PPO] (b and c) were prepared by sequential addition of the monomers. Conversion of glycidol based on the amount of glycidol added. d Number of periphery OH groups generated by arborization of PEO chains and calculated using eq 1. e Average number of allyl glycidyl ether f g units incorporated. Mn calculated based on the amount of the added monomers. Determined using eq 2. sequence formed. We thus propose a one-pot two-step synthetic addition of 0.3 mL/h. The mixture was cooled to room tem- strategy to dendrimer-like PEOs of second and third generation, perature after addition, and the active species were deactivated combining AROP of EO from partially deprotonated multi- by adding a few drops of methanol. The crude solution was first hydroxylated precursors and use of G as branching agent, in a precipitated using an excess of diethyl ether to remove DMSO. semicontinuous addition process. In addition, G was - The viscous polymer obtained was precipitated twice again in ized with allyl glycidyl ether (AGE) so as to introduce allylic cooled diethyl ether from a THF solution. The polymer (4.2 g, double bonds at the branching points of dendrimer-like PEOs. 77%) was collected by filtration and dried at room temperature under vacuum. The data pertaining to the first generation thus synthesized are listed in Table 1. Experimental Section Random Polymerization Procedure. A typical synthetic pro- Materials. Ethylene oxide (EO) (Fluka, 99.8%) was distilled cedure of G1d-[HPGNPPO] is as follows. A two-neck 250 mL over sodium into a buret. Diphenylmethylpotassium (DPMK) flask was charged with lyophilized trihydroxymethylethane was prepared according to known procedures21,22 and titrated precursor (64 mg, 5.3 10-4 mol) and dry DMSO (20 mL). with acetanilide. Glycidol (Aldrich, 99%) was distilled two times DPMK (4.8 10-4 mol) was introduced so as to deprotonate under reduced pressure prior to use. Allyl glycidyl ether, pro- 30% of the hydroxyls of the precursor. Ethylene oxide (2.7 mL, pylene oxide, and dimethyl sulfoxide (Aldrich, 99%) were 54 mmol) was added dropwise onto this solution. The polym- distilled over CaH2 prior to use. All precursors for the polym- erization was carried out at room temperature for 3 days. An erization of EO were dried by freeze-drying from a dioxane aliquot was sampled out for analysis. Glycidol (0.59 g, 8 mmol), solution. In all cases, the PEO derivatives were recovered as allyl glycidyl ether (0.36 g, 3.2 mmol), and propylene oxide (1.1 white powders. All other chemicals were purchased from mL, 16 mmol) in a DMSO solution (10 mL) were added with the Aldrich and used without further purification. Becton Dickinson dosing pump, with a rate of addition of 0.3 Anionic Ring-Opening Polymerization of Ethylene Oxide Fol- mL/h, and the polymerization was carried out at 120 °C under a lowed by Branching Reaction Using Glycidol and Propylene nitrogen atmosphere. The reaction mixture was allowed to stir Oxide. Sequential Polymerization Procedure. Synthesis of for two additional hours after addition of the monomers and the G1b-[HPGNPPO] (Table 1). A two-neck 250 mL flask was flask was cooled to room temperature. The active species were charged with lyophilized 1,1,1-tris(hydroxymethyl)ethane deactivated by adding a few drops of methanol, and the crude (80 mg, 6.7 10-4 mol) and dry DMSO (20 mL). DPMK (6.0 solution was first precipitated in a large excess of diethyl ether to 10-4 mol) was introduced so as to deprotonate ∼30% of the remove DMSO. The viscous polymer obtained was again pre- hydroxyls of the precursor.21,22 EO (3.4 mL, 68 mmol) was cipitated twice in cooled diethyl ether from a THF solution. The added to this solution, and the polymerization was carried out at polymer was collected by filtration and dried at room tempera- room temperature for 3 days. An aliquot was sampled out for ture under vacuum (2.2 g, 52%). The corresponding molecular analysis. Glycidol (0.74 g, 10 mmol) in a DMSO solution (5 mL) characteristics are provided in Table 1. was added with a dosing pump (Becton Dickinson) with a rate of A similar procedure was followed to synthesize the addition of 0.3 mL/h, and the polymerization was carried out at G2a-[HPGN0PPO]. Onto a partially deprotonated G1a-[HPGN- 120 °C under a nitrogen atmosphere. Two hours after adding PPO] precursor (0.98 g, 1.88 10-4 mol) in 20 mL of dry DMSO glycidol, propylene oxide (2.1 mL, 30 mmol) in a DMSO (the extent of deprotonation is equal to 30% using the solution solution (5 mL) was added dropwise with the same rate of of DPMK), ethylene oxide (1.3 mL, 28.6 mmol) was added, and 7294 Macromolecules, Vol. 42, No. 19, 2009 Feng et al.

Scheme 1. Synthetic Pathway to Dendrimer-like PEOs Using Glycidol and Propylene Oxide during the Arborization Stepa

a Green, violet, and red spots represent the central core, the branching points derived from glycidol, and the OH terminal groups, respectively. the polymerization was carried out at room temperature for MALDI-TOF mass spectrometry was performed using a 3 days. A mixture of glycidol (0.40 g, 5.4 mmol) and propylene Micromass TofSpec E spectrometer equipped with a nitrogen oxide (1.2 mL, 16.2 mmol) in DMSO (10 mL) was added at a rate laser (337 nm), a delay extraction, and a reflector. The MALDI of 0.3 mL/h, and the polymerization was carried out at 120 °C mass spectra represent averages over 100 laser shots. This under a nitrogen atmosphere. The polymer was recovered using instrument operated at an accelerating potential of 20 kV. The the same procedure as that described above (2.4 g, 68%). The polymer was first dissolved in THF (10 g L-1). The matrix data pertaining to the second generation of dendrimer-like solution (1,8-dihydroxy-9(10H)anthracenone, dithranol) was PEOs thus synthesized are provided in Table 2. dissolved in THF. The polymer solution (2 μL) was mixed with Characterization. 1H NMR spectra were recorded on a Bru- 20 μL of the matrix solution, and 2 μL of a sodium iodide -1 ker AC 200 spectrometer with CDCl3 as solvent. Size exclusion solution (10 g L in methanol) was added to favor ionization by chromatography (SEC) was performed on a SEC apparatus cation attachment. The final solution (1 μL) was deposited onto fitted with one guard column PLGel 5 μm (50 7.5 mm) and the sample target and dried in air at room temperature. two PLGel 5 μm MIXED-C columns (300 7.5 mm) and a (RI) detector (Jasco, RI-1530) with DMF Results and Discussion (1 g/L LiBr) as eluent (0.8 mL/min) at 60 °C. The apparent - molar masses were calculated using linear poly(ethylene oxide) As discussed in our previous studies,22 26 conditions suitable standards. to polymerize EO from multifunctional hydroxylated precursors Article Macromolecules, Vol. 42, No. 19, 2009 7295

Scheme 2. Effect on the Type of Terminal Hydroxyls after the Arborization Step in the Synthesis of Dendrimer-like PEOs: Statistical vs Sequential Copolymerization of Glycidol and Propylene Oxide

monomer. For instance, G was directly polymerized from a R- hydroxy-ended linear PEO and PEO-b-HPG double hydrophilic block (DHBCs) were successfully synthesized in this way.43 In contrast, Dworak et al. in their attempt to polymerize nonprotected G from cesium alkoxides carried by PEO chain end observed chain transfer of propagating alkoxides to the mono- mer, some HPG homopolymer being formed besides the targeted PEO-b-HPG DHBC.44 The authors therefore resorted to pro- tected G as branching agent to achieve “pom-pom-like” PEOs. In our very first attempts, G was employed alone without propylene oxide (PO) as a comonomer (see further) during the arborization step. The polymerization proceeded as expected, affording a compound denoted G1-[HPGN], where N is the average number of terminal hydroxyl groups. This compound served as precursor of the second generation of PEO, G2, which could be readily achieved after AROP of EO. The SEC trace of G2 indeed exhibited a unimodal and symmetrical distribution. However, when repeating the same sequence of reactions, that is, the AROP of G and of EO in this order, the dendrimer-like PEOs of second Figure 1. SEC traces of dendrimer-like PEOs from the first to the third and third generation exhibited SEC traces of broader molar mass generation. distribution and a shoulder peak in the low molar mass region. Very likely, not all terminal hydroxyls or alkoxides of the are among others a dissociating medium such as dimethyl corresponding precursors were accessible and available for EO sulfoxide (DMSO) and a partial deprotonation of the hydroxyl initiation. This was also observed by Frey and Lutz et al. in their groups (<30%) by a solution of diphenylmethylpotassium attempt to derive multiarm PEO stars from HPGs used as (DPMK). Because the rate of reversible (degenerative) transfer multifunctional macroinitiators.14 To prevent that self-aggrega- of protons between active alkoxides and dormant hydroxylated tion of the growing alkoxides affects the initiation step, these species is much faster than that of propagation,41,42 this partial authors proposed to introduce several hydrophobic PO units deprotonation limits the extent of aggregation. The trifunctional before growing PEO arms.14 We thus adapted this procedure for precursor thus used, namely 1,1,1-tris(hydroxymethyl)ethane (1), the arborization of PEO chain ends, through either the sequential afforded well-defined hydroxy-ended three-armed PEO stars, or the statistical copolymerization of G and PO at 120 °C, using denoted as G1, following a procedure already reported.22-26 DMSO as solvent (Scheme 1). The presence of a few PO units at The key point in our novel synthetic strategy to dendrimer-like PEO chain ends combined with a partial deprotonation of PEOs is the introduction of a few glycidol monomer units at the hydroxyls (30%) indeed minimized the self-aggregation of alk- PEO chain ends. Previous reports have described that glycidol oxides, which proved helpful to grow the next generation of PEO (G) can be polymerized under controlled conditions by slowly under controlled/living conditions. As one G generates two adding the monomer to a partially deprotonated (10%) multi- hydroxyls after ring-opening, two G monomer units generate hydroxy-containing core initiator.33 The concentration of G has three hydroxyls in total and so on: the average number of to be low enough, however, to avoid the transfer reaction that terminal hydroxyls per PEO arm (N(OH)per arm) of the targeted may occur from active chain ends to the hydroxyl carried by the structure, denoted G1-[HPGNPPO], thus corresponded to the 7296 Macromolecules, Vol. 42, No. 19, 2009 Feng et al.

1 Figure 2. H NMR spectrum (CDCl3 þ trifluoroacetic anhydride) of the second generation G2b dendrimer-like PEO and its precursor G1b-[HPGNPPO], allowing for the determination of the content in terminal hydroxyls (Table 1).

molecular features of all PEO dendrimer-like samples prepared and their precursors. No major difference is observed between the dendritic PEO samples for which branching points were gener- ated from the sequential or the random copolymerization of G and PO. The only difference is actually the type of hydroxyls present at PEO chain ends of the precursor, as shown in Scheme 2: only secondary hydroxyls are formed when G and PO are sequentially copolymerized during the arborization step, whereas both primary and secondary hydroxyls result from the random copolymerization of the two monomers. Molar masses as well as the average number of peripheral hydroxyl groups of the PEO derivatives could be determined by 1H NMR spectroscopy. In particular, the content in hydroxyl groups was evaluated after their derivatization into trifluoroe- sters using trifluoroacetic anhydride. In Figure 2, for instance, the 1 H NMR spectrum of G1-[HPGNPPO] precursor of the dendrimer-like PEO of second generation, G2b, shows the signals of both the methylene (-CH2O-) and the methine (-CH(CH2O-)OH-) protons due to the resonance of the primary and the secondary derivatized trifluoroesters at 4.48 and 5.22 ppm, respectively, without interference with other Figure 3. 1H NMR characterization of the second generation G2d signals. dendrimer-like PEO (B) and its precursor G1d-[HPGNPPO] The methyl protons of the core still can be detected at (A) synthesized using glycidol, propylene oxide, and allyl glycidyl ether 0.90 ppm. Signals due to the protons of PO units appear at during the arborization step. 1.32 and 1.13 ppm for methyl protons, whereas methylene and methine protons overlap with the PEO signal from 3.7 to 3.4 ppm. total amount of G polymerized (NG) plus one: N(OH)per arm = NG One can also note that the two signals of the methyl protons of þ1 (see Scheme 2). The same two-step sequence of polymeriza- PO units merged as one peak at 1.17 ppm for G2b. This is likely tion of EO followed by arborization of PEO chain ends through due to the change in the chemical environment of the methyl copolymerization of G with PO was repeated to grow the second protons before and after the growth of PEO arms of the second PEO generation, affording a dendrimer-like sample, noted G2- generation: the precursor indeed carried trifluoacetic ester groups HPGN0PPO. In the same way, a third generation of PEO was attached to the asymmetric carbon of the terminal PO unit, grown, affording G3 with an average number molar mass up to whereas in the second generation dendrimer-like PEO the same 55 000 g/mol and a polydispersity of 1.31 (Scheme 1 and Table 2). ester linkages are separated with ether ones. The ratio of As shown in Figure 1, narrow SEC traces were obtained from integration of the peaks at 4.48 and 5.22 ppm mentioned above the first to the third generation. In Tables 1 and 2 are summarized to that of the peak due to the methyl protons of the core at Article Macromolecules, Vol. 42, No. 19, 2009 7297

Figure 4. MALDI TOF mass spectrum of G1a-[HPGNPPO]. 0.90 ppm helped us to deduce the number and the type of 120, 56, 44, and 74 are the molar mass of 1,1,1-trihydroxymethy- hydroxyl groups (N number in Table 1). lethane, PO, EO, and glycidol, respectively (Tables 1 and 2). When randomly copolymerized during the arborization step, Finally, functional groups could be introduced at the interior G and PO are expected to generated two types of randomly of dendrimer-like PEOs using a functional comonomer, namely, distributed terminal hydroxyls (Scheme 2). As anticipated, two allyl glycidyl ether (AGE), during the arborization step. The broad peaks attributed to the corresponding trifluoroesters incorporation of functional groups at the interior of regular appeared around 4.60 and 5.30 ppm (Figure 2). Upon initiation dendrimers has already been exploited to finely tune their of EO from the two types of deprotonated hydroxyls, however, physical properties and target specific applications or to favor only primary alkoxides are generated. The total number of intramolecular reaction by a concentration effect.45 In addition, hydroxyls thus calculated after both arborization and growth we have recently reported the first example of poly(acrylic acid)- of the second generation of PEO chains remains constant, containing dendrimer-like PEOs that were obtained through the indicating that initiation took place from all of the hydroxyls functionalization of their interior with allylic groups using an 24 (Tables 1 and 2). In this way, the number of terminal OH and AB2C-type branching agent. In the present work, the three hence the number of arms of each generation of PEO could be monomers (G, PO, and AGE) were slowly added onto the living adjusted upon varying the amount of G added. alkoxides of the PEO arm ends, in order to generate both allyl- From 1H NMR characterization and precisely, from the peak containing branching junctions and the hydroxyl groups required due to trifluoroesters, the number of peripheral hydroxyl groups to grow the next generation. Average numbers of three and five is calculated using the following equation: allyl groups were respectively introduced (Tables 1 and 2). Atypical1H NMR spectrum of a functionalized dendrimer-like NðOHÞ ¼ð3I5:3=I0:9Þþð3I4:5=2I0:9Þð1Þ PEO is shown in Figure 3. The peaks characteristic of the allylic protons (f, g, h) can be where I5.3 and I4.5 represent the signal intensity of terminal seen at 5.85, 5.25, and 4.08 ppm, respectively. These double bonds methylene and methine protons connected to trifluoro groups, can be readily derivatized into other functional groups as 24 respectively, and I0.9 is the intensity of the core methyl signal at reported previously. 0.90 ppm. The total molar mass of the PEO is thus equal to the Characterization by MALDI TOF mass spectroscopy of these sum of the molar mass of the three constitutive units, PO, G, and branched PEOs is a challenging task owing to the presence of EO, according to eq 2: three types of monomer units (EO, G, and PO) whose number can fluctuate from one branched macromolecule to another. In MnðNMRÞ ¼ MnðPPOÞþMnðPGlyÞþMnðPEOÞ ð2Þ addition, it is important to note that in the present case the molar with mass of three EO units coincidently equals the molar mass of one ¼ ð = Þ MnðPPOÞ 56 I1:3-1:1 I0:9 G unit plus that of one PO unit: 3 EO = G þ PO = 132 g mol-1. This obviously complicates further the MALDI TOF M ð Þ ¼ 74ðNð Þ -3Þ n PGly OH mass spectra of these PEO derivatives, and therefore the data interpretation should be made with caution. A representative MnðPEOÞ ¼ 120þ44 3½I3:6 -I1:3-1:1 -5ðNðOHÞ -3ÞI0:9=3=4I0:9 MALDI TOF spectrum of the G1a-[HPGNPPO] sample is shown in Figure 4. First, the parent three-arm PEO star that served as I1.3-1.1, I0.9,andI3.6 being the integral of the peaks due to PO precursor to G1a-[HPGNPPO] before addition of G and PO units, the initiating core, and EO units, respectively; the values should give, if present, a main peak in the enlarged region 7298 Macromolecules, Vol. 42, No. 19, 2009 Feng et al. appearing at m/z = 4460.60. This population being not observed, (5) Hiki, S.; Kataoka, K. Bioconjugate Chem. 2007, 18, 2191–2196. one can assume that all of the three hydroxyls of this parent PEO (6) Vadala, M. L.; Thompson, M. S.; Ashworth, M. A.; Lin, Y.; have been utilized during the arborization step, as also supported Vadala, T. P.; Ragheb, R.; Riffle, J. S. Biomacromolecules 2008, – by the 1H NMR monitoring discussed above. Next, the different 9, 1035 1043. (7) For a very recent review on star-shaped polymers with PEO arms, series of peaks shown in the enlarge region of Figure 4 can be see: Lapienis, G. Prog. Polym. Sci. 2009, ASAP; doi: 10.1016/j. explained as follows. For example, the peak at m/z = 4457.16 g progpolymsci.2009.04.006. -1 mol is likely the result of the following structure: EO88G2PO5 (8) Fee, C. J. Biotechnol. Bioeng. 2007, 98, 725–731. whose molar mass equals 88 44.05 þ 2 74.08 þ 5 58.08 þ (9) Gasteier, P.; Reska, A.; Schulte, P.; Salber, J.; Offenhausser, A.; 120 þ 23, where 120 and 23 are the molar mass of the trifunctional Moeller, M.; Groll, J. Macromol. Biosci. 2007, 7, 1010–1023. core precursor and of sodium ion, respectively, 44.05, 74.08, and (10) Taton, D.; Saule, M.; Logan, J.; Duran, R.; Hou, S.; Chaikof; E. L.; Gnanou, Y. J. Polym. Sci., Part A: Polym. Phys. 2003, 41, 1669– 58.08 being the molar mass of EO, G, and PO, respectively. 1676. Similarly, the peaks at m/z = 4471.13 and m/z = 4485.14 could (11) Hou, S.; Taton, D.; Saule, M.; Logan, J.; Chaikof, E. L.; Gnanou, be attributed to the structures EO87G2PO6 and EO87G3PO5, Y. Polymer 2003, 44, 5067–5074. respectively. (12) Lapienis, G.; Penczek, S. Biomacromolecules 2005, 6, 752–762. In reality, a particular ion peak could be ascribed to different (13) Lapienis, G. J. Polym. Sci., Part A: Polym. Phys. 2007, 45, 5017– combinations of the three monomer units. For example, the peak 5021. -1 (14) Knischka, R.; Lutz, P. J.; Sunder, A.; Mulhaupt, R.; Frey, H. mentioned above at m/z = 4457.16 g mol can also result from Macromolecules 2000, 33, 315–320. the following combinations: EO91G1PO4 or EO85G3PO6 or (15) Pfister, A.; Fraser, C. L. Biomacromolecules 2006, 7, 459–468. EO82G4PO7 or EO79G5PO7, etc. However, if one considers that (16) Hawker, C. J.; Chu, F.; Pomery, P. J.; Hill, D. J. T. Macromolecules this peak does derive from a parent three-arm star with 88 EO 1996, 29, 3831–3838. units and since the NMR results gave an average number of 5.5 (17) Unal, S.; Lin, Q.; Mourey, T. H.; Long, T. E. Macromolecules 2005, – terminal OH corresponding to 2.5 units of G polymerized, one 38, 3246 3254. (18) Lapienis, G.; Penczek, S. Macromolecules 2000, 33, 6630–6632. can reasonably conclude that the most probable combinations (19) Lapienis, G.; Penczek, S. J. Polym. Sci., Part A: Polym. Chem. for this ion peak are EO88G2PO5,EO88G2PO6, and EO88G3PO5. 2004, 42, 1576–1598. (20) Walach, W.; Trzebicka, B.; Justynska, J.; Dworak, A. Polymer Conclusion 2004, 45, 1755–1762. (21) Six, J.-L.; Gnanou, Y. Macromol. Symp. 1995, 95, 137–150. We propose a fast and versatile two-step synthetic strategy to (22) Feng, X.-S.; Taton, D.; Chaikof, E. L.; Gnanou, Y. J. Am. Chem. dendrimer-like poly(ethylene oxide)s (PEOs) with high loading Soc. 2005, 127, 10956–10966. capacity that relies on an iterative divergent method, combining (23) Van Renterghem, L. M.; Feng, X.; Taton, D.; Gnanou, Y.; Du two elementary steps that are (i) the anionic ring-opening polym- Prez, F. E. Macromolecules 2005, 38, 10609–10613. (24) Feng, X.; Taton, D.; Chaikof, E. L.; Gnanou, Y. J. Am. Chem. Soc. erization of ethylene oxide (AROP of EO) and (ii) the semicon- 2006, 128, 11551–11562. tinuous and slow addition of a commercially available branching (25) Feng, X. S.; Taton, D.; Chaikof, E. L.; Gnanou, Y. Biomacro- agent, namely nonprotected glycidol, during the arborization step molecules 2007, 8, 2374–2378. of PEO chain ends. Random or sequential copolymerization of (26) Feng, X.-S.; Taton, D.; Ibarboure, E.; Chaikof, E. L.; Gnanou, Y. propylene oxide with glycidol, in addition to a low degree of J. Am. Chem. Soc. 2008, 130, 11662–11676. deprotonation of hydroxyl groups, and use of DMSO as a solvent (27) Singh Danhikula, R.; Hidgen, P. Bioconjugate Chem. 2006, 17, 29–41. are the appropriate conditions to avoid the formation of aggre- (28) Newkome, G. R.; Kotta, K. K.; Mishra, A.; Moorefield, C. N. gates during the AROP of EO from multihydroxylated precur- Macromolecules 2004, 37, 8262–8268. sors. A series of dendrimer-like PEOs up to generation 3 with an (29) Fernandez-Megia, E.; Correa, J.; Riguera, R. Biomacromolecules average number molar mass up of 55 000 g/mol and a polydis- 2006, 7, 3104–3111. persity of 1.31 could thus be synthesized. This strategy does not (30) Taton, D.; Feng, X.; Gnanou, Y. New J. Chem. 2007, 31, require any protection/deprotection step, and only an intermedi- 1097–1110. (31) Hirao, A.; Sugiyama, K.; Tsunoda, Y.; Matsuo, A.; Watanabe, T. ate purification by precipitation is necessary before growing the J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6659–6687. next generation of PEO. Alternatively, allyl glycidyl ether can be (32) Vandenberg, E. J. J. Polym. Sci., Part A: Polym. Chem. 1985, 23, readily copolymerized with propylene oxide and glycidol in order 915–949. to introduce allylic groups during the arborization step; this offers (33) Sunder, A.; Hanselmann, R.; Frey, H.; Mulhaupt, R. Macromole- possibilities for further functionalization/derivatization of the cules 1999, 32, 4240–4246. interior of these dendrimer-like PEOs. Thus, advantages provided (34) Sunder, A.; Turk, H.; Haag, R.; Frey, H. Macromolecules 2000, 33, 7682–7692. by the dendritic scaffold, in particular its multivalency due to the (35) Sunder, A.; Mulhaupt, R.; Haag, R.; Frey, H. Macromolecules presence of numerous hydroxyl terminal groups, can be easily 2000, 33, 253–254. combined with the very unique features of PEO. (36) Sunder, A.; Mulhaupt, R.; Frey, H. Macromolecules 2000, 33, 309–314. Acknowledgment. The authors are grateful to the Centre (37) Frey, H.; Haag, R. Rev. Mol. Biotechnol. 2002, 90, 257–267. National de Recherches Scientifique (CNRS), the French (38) Barriau, E.; Pastor-Perez, L.; Berger-Nicolletti, E.; Kilbinger, A. F. Ministry of High Education, and a NIH grant (program 5R01 M.; Frey, H.; Stiriba, S.-E. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2049–2061. RR14190-04) that helped support X.F. (39) Tokar, R.; Kubisa, P.; Penczek, S. Macromolecules 1994, 27, 320–322. References and Notes (40) Erberich, M.; Keul, H.; Moeller, M. Macromolecules 2007, 40, 3070–3079. (1) Thompson, M. 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